1. Field of the Invention
The present invention relates to a thin film transistor substrate, and more particularly to a thin film transistor substrate in which the heat generation by optical incidence is inhibited. The invention also relates to a method of manufacture of the thin film transistor substrate.
2. Description of Related Art
In a liquid crystal display using a thin film transistor, to increase the number of picture elements, the miniaturization of electrodes and wiring is increasing. Therefore, the use of metal mainly composed of Al with low electric resistance for the electrode material has become conventional.
However, when a picture element electrode made of indium tin oxide (ITO) is to be connected to an electrode made of a metal mainly composed of Al, There may be insufficient contact. This failure is caused by an insulating oxide film that is formed on the surface of the electrode made of the metal mainly composed of Al by natural oxidation or by an oxidized atmosphere in a chamber when the ITO is sputtered.
To make a contact properly between the electrode made of the metal mainly composed of Al and the picture element electrode made of ITO, for example, Japanese Patent Laid-Open No. JP-A-10-65174 (patent document 1) discloses a high-melting point metal such as Ti, W, Mo, Ta and Cr is laminated on the metal mainly composed of Al in a thin film transistor. These high-melting point metals can cause a satisfactory contact with ITO because they are conductive even if the surface is oxidized. In addition, in a thin film transistor disclosed in Japanese Patent Laid-Open No. JP-A-9-307113 (patent document 2), satisfactory contact with ITO is achieved by laminating Ti and TiN on metal mainly composed of Al.
FIG. 12 is a sectional view showing conventional type thin film transistor substrate. Referring to this drawing, the conventional type thin film transistor substrate will be described below.
A base insulating film 82 formed by a SiO2 film is formed on transparent base substrate 81 made of glass or suitable material. Thin film transistor 96 includes active layer that is made of polycrystalline silicon and that includes a drain region 83, a channel region 84 and a source region 85. The active layer is formed on the base insulating film 82 in the shape of an island. Impurities such as phosphorus are added to the drain region 83 and the source region 85 to reduce resistance. A gate insulating film 86 formed by a SiO2 film is formed on the active layer. A gate electrode 87 made of metal, metal silicide or polysilicon to which impurities are added is formed over the channel region 84 with the gate insulating film 86 between the gate electrode and the channel region.
A first insulating interlayer 88 formed of a SiO2 film or other suitable material is formed on the gate electrode 87 so that the first insulating interlayer covers the entire surface. A drain electrode 89 and a source electrode 90 respectively made of metal mainly composed of Al or suitable material is formed on the first insulating interlayer 88. The drain electrode 89 and the source electrode 90 are connected to the drain region 83 and the source region 85 via contact holes 97 made in the first insulating interlayer 88 and the gate insulating film 86.
A second insulating interlayer 91 formed of a SiO2 film or a silicon nitride film or suitable material is formed on the drain electrode 89 and the source electrode 90 so that the second insulating interlayer covers the overall surface. An upper electrode 92 is formed on the second insulating interlayer 91 so that the upper electrode covers the active layer of the thin film transistor 96 to prevent light from being incident on the active layer of the thin film transistor 96. The upper electrode 92 is composed of a lamination of an upper high-melting point metal layer 92b made of Ti, W, Mo, Ta or Cr whose contact with ITO is satisfactory and a second Al metal layer 92a mainly including Al. The upper electrode 92 is connected to the source electrode 90 via a contact hole 98 made in the second insulating interlayer 91.
A third insulating interlayer 93 formed by a SiO2 film or a silicon nitride film and others and a flattening film 94 made of acrylic resin are formed on the upper electrode 92 so that the third insulating interlayer 93 and the flattening film 94 cover the overall surface. A picture element electrode 95 made of ITO is formed on the flattening film 94. The picture element electrode 95 is connected to the high-melting point metal layer 92b of the upper electrode 92 via a contact hole 99 made in the flattening film 94 and the third insulating interlayer 93.
FIG. 13 is a sectional view showing a liquid crystal display using the above-described conventional type thin film transistor substrate. A thin film transistor substrate 80 where multiple thin film transistors 96 are formed is bonded to an opposite substrate 102 on which an opposite electrode 101 made of ITO with a predetermined separation. A liquid crystal display 100 is formed by filling the separation with a liquid crystal 103. In case the liquid crystal display 100 is used for a liquid crystal projector using a high-intensity light source, there is a problem that unevenness occurs in a projected image because the thin film transistor substrate 80 is distorted by heat caused during projection, which causes an interval between the picture element electrode 95 and the opposite electrode 101 to vary which cause the dispersion to increase.
In the liquid crystal display 100 used for the liquid crystal projector, generally, projected light P is incident from the side of the opposite substrate 102. Apart of the incident light reaches the upper electrode 92 formed in the thin film transistor substrate 80. The upper layer of the upper electrode 92 includes the high-melting point metal layer 92b. The reflectance of layer 92b is approximately 70% at most and is smaller, compared with an Al alloy showing the reflectance of approximately 90%. Therefore, the high-melting point metal layer 92b absorbs a part of the light that reaches the upper electrode 92 and as a result, heat increases.